Feeding the infant gut microbes

Early life exposures like breastfeeding influence the development of full-term and preterm infants. Benefits of breast milk on infants are hypothesised to be mediated by milk bacteria and human milk oligosaccharides (HMO) on the developing infant gut microbiota. In this thesis, I studied how milk bacteria and HMOs affect infant gut microbiota development and health. For my research, I used data from two cohorts, the Dutch mother-infant cohort ‘Lifelines NEXT’ and the Swedish, randomised, placebo-controlled probiotic intervention trial ‘Prophylactic Probiotics to Extremely Low Birth Weight Premature Infants’. First, I present optimised protocols to study low-biomass milk and preterm infant faecal microbiota. Then, I show that the milk microbiota has a low diversity, differs from faecal microbiota, and changes during lactation. Milk bacteria can be shared with and modify (preterm) infant gut microbiota early in life. Furthermore, I characterise the HMO composition in breast milk and describe strong effects of maternal genetics and the time postpartum on HMOs. Like milk bacteria, also HMOs influence infant gut microbiota. In addition, infant gut bacteria are linked to immune system education and specific HMOs and bacteria are weakly associated with infant health outcomes, such as growth. In summary, this thesis provides a deep characterisation of breast milk microbiota and HMO composition and their influences on developing infant gut microbiota and infant health. My findings contribute to a better understanding of the beneficial effects of breast milk and will help to improve maternal and infant care, e.g., via personalised advice and interventions.